1. Field of the Invention
The present invention relates to an integrated circuit pattern lithography system and its method for forming pattern data of an integrated circuit, and more particularly to an integrated circuit pattern lithography system and its method capable of improving in throughput by forming patterns at once partially in an area where the same patterns are repeatedly formed through charged particle beam exposure with a preliminary transparent mask.
2. Description of the Related Art
Recently, circuit patterns are getting finer and finer in order to realize large scale integration in the manufacturing technique of a semiconductor device. A pattern exposure system with an electron beam or the other charged particle beam which can form finer patterns is being used instead of the conventional optical pattern exposure. In the optical pattern exposure system, a mask with a pattern of an integrated circuit chip formed on a quartz glass is made and a given area is exposed at once with this mask, so to form integrated circuit patterns on a semiconductor substrate, thereby featuring high throughput. On the contrary, the charged particle beam exposure system is unable to do a batch exposure of chip patterns, differently from the optical pattern exposure system, but it is formed each pattern in a way of direct writing a line in every pattern, or, what is called, with a single stroke of the brush. Therefore, according as an integrated circuit becomes larger scale integration and the patterns to be formed are more increased in number, it takes much more time to be formed patterns on the whole chips, thereby deteriorating the throughput.
Then, proposed is an exposure system having a mask with a plurality of transparent holes for forming a repeat pattern unit (hereinafter, referred to as a cell projection mask) as illustrated in FIG. 13(a), by the use of the characteristic that many of the basic circuits are formed by the repeat of the same pattern in the case where an integrated circuit chip 211 is an integrated circuit chip including a semiconductor memory. The exposure system performs pattern forming by exposing a repeat pattern area 201 of FIG. 13(a) to a charged particle beam through a cell projection mask. In an irregular pattern area 213, patterns are formed in the conventional forming method with a single stroke of the brush. In these ways, it can shorten the direct writing time extremely on the whole as well as improve in throughput.
Design data of the repeat pattern area 201 in a semiconductor integrated circuit is formed by expansion of the minimum repeat units 202 in array, as illustrated in FIG. 13(b). The size of the minimum repeat unit 202 depending on a chip, the description will be made, by way of example, in the case of 1 Gb (gigabit) DRAM (Dynamic Random Access Memory). The most typical area as the repeat pattern area of DRAM is a memory cell array area. In the case of 1 Gb DRAM, there exist about one billion memory cells. Some of these memory cells gather in an array to form the repeat pattern area 201, and the respective repeat pattern areas 201 are dispersedly disposed on the chip.
The size of one memory cell for use in 1 Gb DRAM is about 0.8 .mu.m.times.0.4 .mu.m. In order to form the design data of the repeat pattern area 201, the minimum repeat unit 202 is formed by four (=2.times.2) memory cells and the minimum repeat units 202 are expanded in array. Namely, the size of the minimum repeat unit 202 is about 1.6 .mu.m.times.0.8 .mu.m. The allowed size 205 of the cell projection mask for use in the partial batch exposure method is generally about 5 .mu.m.times.5 .mu.m. Therefore, since the size of the minimum repeat unit 202 is smaller than the allowed size 205 of the cell projection mask as illustrated in FIG. 13(c), the same minimum repeat unit 202 may be used as the cell projection mask. As a result, the repeat pattern area 201 has an expansion array of the minimum repeat units 202 in 16 rows and 32 columns as illustrated in FIG. 13(b). If the cell projection exposure method may adopt a cell projection mask all based on the minimum repeat units 202, every pattern could be generated by the exposure of "512" (=16.times.32) shots.
The design pattern 203 illustrated within the minimum repeat unit 202 of FIG. 13(c) is one example of a pattern for forming a field and one minimum repeat unit 202 includes three design patterns 203 as illustrated. When the design pattern 203 included in the minimum repeat unit 202 is formed by the conventional pattern lithography with a variable rectangular beam, since the design pattern 203 is formed by dividing it into individual five rectangles, the necessary shot number for pattern forming on one minimum repeat unit 202 becomes five. Therefore, in the whole repeat pattern area 201, every pattern can be formed by the number "2560" (=16.times.32.times.5) shot exposure. Namely, in the example of FIG. 13(c), the cell projection exposure method can reduce the shot number into one fifth of the conventional variable shaped beam exposure method, thereby improving in throughput. Since one part of the design pattern 203 illustrated in FIG. 13(c) is sequential between both adjacent minimum repeat units 202 in a horizontal direction, strictly speaking, the shot number by the conventional variable shaped beam exposure method would be not more than five times as many as the number of the repeat units 202. However, for brevity's sake, the description hereinafter will be made on the assumption that the number of the exposure shots by the conventional variable shaped beam exposure method would be five times as many as the number of the repeat units 202.
This conventional method, however, cannot make effective use of the allowed size 205 and insufficient to the improvement in throughput, because the size of the minimum repeat unit 202 is extremely smaller than the allowed size 205 of the cell projection mask.
In order to improve the throughput more effectively, it is considered that the repeat unit is fixed at the larger size than the minimum repeat unit 202. With reference to FIG. 14, the improving method will be described. FIG. 14(b) shows a maximum repeat unit 206 as the repeat unit for use in the cell projection mask, in which the minimum repeat units 202 are arrayed as many as possible within the range of the allowed size 205 of the cell projection mask. With reference to FIG. 14(a), the maximum repeat unit 206 for use in the cell projection mask has an expansion array of the minimum repeat units 202 in 3 rows and 6 columns. Therefore, the size of the maximum repeat unit 206 is 4.8 .mu.m.times.4.8 .mu.m.
The repeat pattern area 201 has an array of the minimum repeat units 202 in 16 rows and 32 columns as mentioned above. Therefore, the repeat pattern area 201 has enough size to include the partial batch exposure area 214 formed by an expansion array of the maximum repeat units 206 in 5 rows and 5 columns, as illustrated in FIG. 14(a). As a result, patterns in the range of the partial batch exposure area 214 can be formed with the shot number "25" by use of the cell projection mask of the maximum repeat unit 206.
However, there remains an area where the maximum repeat units 206 cannot be expanded, within the repeat pattern area 201, as illustrated. The remaining area is in the size for 62 (=16.times.32-18.times.25) minimum repeat units 202 in this example. Then, the conventional variable shaped beam exposure method is adopted to the remaining area, as the variable shaped beam exposure area 215, in order to form the design pattern 203 there.
This operation, however, cannot reduce a sufficient number of shots. More specifically, the shot number becomes "310" (=62.times.5) in the remaining area for the data processing as the variable shaped beam exposure area 215. The total shot number becomes "335". As mentioned above, the shot number in the partial batch exposure area 214 can be enormously reduced, while the shot number in the variable shaped beam exposure area 215, however, becomes twelfth times or more than the shot number in the partial batch exposure area 214, which is not a sufficient reduction although the shot number is reduced compared with the case of the minimum repeat units used for the cell projection mask.
Since two types of exposure methods, partial batch exposure and variable shaped beam exposure, are used in order to form the design patterns 203 in the whole repeat pattern area 201, the pattern shape and position of the partial batch exposure area 214 is completely different from the pattern shape and position of the variable shaped beam exposure area 215. When the shape and position differs in the exposure pattern in the repeat pattern area 201, the characteristics of the individual semiconductor devices to be formed by these patterns also differ from each other. As a result, reliability and yield of the integrated circuit would be deteriorated.